I don't know where I first ran across this rule of thumb, but I've allways equated 1 ton of alfalfa to 25 bushels of corn. 

If 25 bushels of corn is selling for $4 per bushel, you better be recieving $100 per ton for the hay or it is underpriced/ or you're better off with corn.  (of course prices change throughout the year)

It also means you have to grow a ton of hay for every 25 bushels of corn that ground is capable of.  If your ground is capable of 150 bushel corn average but can only grow 4 tons alfalfa, you're better off with corn.

Spray the alfalfa, skip the nitrogen next year, no-till the corn and the the old hay stand will pay its dividends with next year's corn.